U.S. patent application number 09/989799 was filed with the patent office on 2002-06-06 for robust diameter-controlled optical fiber during optical fiber drawing process.
Invention is credited to Wang, Sheng-Guo.
Application Number | 20020066292 09/989799 |
Document ID | / |
Family ID | 26941273 |
Filed Date | 2002-06-06 |
United States Patent
Application |
20020066292 |
Kind Code |
A1 |
Wang, Sheng-Guo |
June 6, 2002 |
Robust diameter-controlled optical fiber during optical fiber
drawing process
Abstract
There is provided robust diameter-controlled optical fiber
during optical fiber drawing process and an optical fiber drawing
process which comprises drawing the optical fiber from a perform
therefor under tension to form the optical fiber while heating and
melting the preform, wherein an outer diameter of the preform is
measured (at a safe position immediately) above the furnace, an
outer diameter of the optical fiber on which no coating has been
provided is measured at one process position or two process
positions before coating, and drawing conditions are robustly
controlled based on the deviation of the measured optical fiber
diameter data and the measured preform diameter data from a
preselected outer diameter of the optical fiber and a preselected
outer diameter of the preform.
Inventors: |
Wang, Sheng-Guo; (Charlotte,
NC) |
Correspondence
Address: |
Dr. Sheng-Guo Wang
2516 Radrick Ln
Charlotte
NC
28262
US
|
Family ID: |
26941273 |
Appl. No.: |
09/989799 |
Filed: |
November 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60250962 |
Dec 4, 2000 |
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Current U.S.
Class: |
65/382 |
Current CPC
Class: |
C03B 37/0253 20130101;
C03B 2205/40 20130101; Y02P 40/57 20151101 |
Class at
Publication: |
65/382 |
International
Class: |
C03B 037/07 |
Claims
What I claim as my invention is:
1. A drawing process for producing an optical fiber comprising the
steps of: measuring the outer diameter/shape of a preform; heating
and melting said preform for said optical fiber; while heating and
melting, drawing said optical fiber from said preform under tension
to form said optical fiber; and providing a control system with the
measured outer diameter/shape data of said preform to control said
drawing process; whereby said optical fiber drawing process will be
robustly controlled with high performance of said process and high
quality of said optical fiber, making a robust diameter-controlled
optical fiber against various deviations of said preform
diameter/shape.
2. The process for producing said optical fiber as claimed in claim
1, wherein the measurement of said preform outer diameter/shape is
either on-line or off-line; and in the former the measured data are
on-line real-time fed to said control system while in the latter
the measured data are pre-stored in said control system, for
controlling said process in face of the deviations of said preform
diameter/shape.
3. The process as claimed in claim 1, wherein said measured outer
diameter/shape data of said preform are fed to said control system
to control the process, including all or either one of the
following controls: a feeding speed control of said preform, a
drawing speed control of said optical fiber, and a tension control
of said optical fiber.
4. The process as claimed in claim 1, further including the steps
of: measuring the outer diameter of said optical fiber at a safe
position below and close to the heating and melting device, in
which the size of said preform is changing substantially to form
said optical fiber by drawing; and providing said control system
with the measured outer diameter data of said optical fiber to
control said drawing process; whereby to maintain robustly high
performance of said drawing process and to provide robustly high
quality of said optical fiber in presence of the deviations of said
outer diameter and shape of said preform.
5. The process as claimed in claim 1, further including the steps
of: measuring the outer diameter of said optical fiber as it is
being drawn at a position at which shrinkage of the outer diameter
of said optical fiber is within a small percentage; providing said
control system with the measured outer diameter data of said
optical fiber; and the drawing being carried out at a drawing rate
that is controlled based on the measurement data and the deviations
of the measured preform outer diameter and the measured optical
fiber outer diameter from their respective preselected outer
diameters.
6. The product made by the process for producing said optical fiber
in claim 1.
7. A drawing process for producing an optical fiber comprising the
steps of: heating and melting a preform for said optical fiber;
while heating and melting, drawing said optical fiber from said
preform under tension to form said optical fiber; after heating and
melting, coating said optical fiber while drawing; measuring the
outer diameter of said optical fiber just before said coating
stage, called a final outer diameter of a bare fiber which is said
optical fiber before coating; and providing a control system with
the measured final outer diameter data of said bare fiber for
controlling said drawing process; whereby to maintain robustly high
performance of said drawing process and to provide the robustly
fine quality of said optical fiber, including a real control
quality of said final outer diameter of said bare fiber, which is
the key quality of said optical fiber.
8. The process as claimed in claim 7, wherein said control system
includes at least one of the following controls, a feeding speed
control of said preform, a drawing speed control of said optical
fiber, and a tension control of said optical fiber.
9. The process as claimed in claim 7, further including the steps
of: measuring the outer diameter of said preform before said
heating and melting; and providing said control system with the
measured outer diameter data of said preform and the final outer
diameter data of said bare fiber to control said drawing process
for robustly controlled high performance of said optical fiber
drawing process and said optical fiber.
10. The process as claimed in claim 7, further including the steps
of: measuring the outer diameter of said bare fiber at a position
below and close to the heating and melting device in which the size
of said preform is changing substantially to form said optical
fiber by drawing; and providing said control system with the outer
diameter data of said bare fiber close to said heating device and
the final outer diameter data of said bare fiber close to said
coating stage to control said drawing process.
11. The robust diameter-controlled optical fiber made by the
process in claim 7.
12. A drawing process for producing an optical fiber comprising the
steps of: heating and melting a preform for the optical fiber;
while heating and melting, drawing said optical fiber from said
preform under tension to form said optical fiber; after heating and
melting, coating said optical fiber while drawing; measuring the
outer diameters of said optical fiber at two or more locations
before the coating stage; and providing a control system with the
measured outer diameter data of said optical fiber from said two or
more locations to control said drawing process; whereby to maintain
robustly controlled high performance of said optical fiber drawing
process and high quality of said optical fiber.
13. The process as claimed in claim 12, wherein the measurements
are after said heating and melting and include; measuring the outer
diameter of said optical fiber at a safe position below and close
to the heating device with a limited delay from the heating and
melting stage; and measuring the outer diameter of said optical
fiber, as it is being drawn, at a second position at which
shrinkage of the outer diameter of said optical fiber is within a
small percentage; and the drawing is carried out at a drawing rate
that is controlled by said control system based on the deviations
of the measurement data from their respective preselected data;
whereby to maintain a control quality of a final outer diameter of
said optical fiber before coating.
14. The process as claimed in claim 12, further including a
measurement of the outer diameter of said preform above the heating
device; and providing said control system with the measured outer
diameter data of said preform and the outer diameter data of said
optical fiber at said two or more locations to control said drawing
process for robustly controlled high performance of said optical
fiber drawing process and said optical fiber.
15. The process as claimed in claim 14, wherein said two or more
locations include: one at a safe position below and close to the
heating and melting device; and another just before the coating
stage, therein the measured outer diameter of said optical fiber is
called a final outer diameter of a bare fiber which is said optical
fiber before coating; whereby to have a real control quality of
said final outer diameter of said bare fiber, which is the key
quality of said optical fiber.
16. The product made by the process in claim 12.
17. A control method for an optical fiber manufacturing process
control comprising the steps of: measuring the objects of the
manufacturing process by more than one measurement devices which
locate at different locations including before and after a stage or
an action therein said object changes either its geometrical shape
or size, or physical property, or chemical property substantially;
analyzing the measurement data from the measurement devices; and
generating a control signal or control signals to control the
optical fiber manufacturing process based on the data and the
analysis; whereby to get robust high performance of the
process.
18. The control method in claim 17, wherein said optical fiber
manufacturing process control is an optical fiber drawing process
control and said measurement devices include: one for the
measurement of a preform before a heating and melting stage, in
which the preform is changing its geometrical size substantially to
form said optical fiber by drawing; and another for the measurement
of said optical fiber after said heating and melting stage; whereby
to robustly control high performance of said drawing process and
high quality of said optical fiber by controlling the preform
feeding speed and the optical fiber drawing speed based on the
measurement data.
19. The control method in claim 17, wherein said optical fiber
manufacturing process control is an optical fiber drawing process
control and said measurement devices include: one after and near
the heating and melting device in which a preform is passing to
form said optical fiber by drawing; and another after the heating
and melting device but near a coating stage; in view of shrinkage
of an outer diameter of said optical fiber between these two
measurement locations; whereby to robustly control high performance
of said drawing process and high quality of said optical fiber
diameter based on the measurement data.
20. The product made by said control method in claim 17.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to optical fibers and a
process for optical fiber drawing.
[0003] 2. Description of the Related Art
[0004] Optical fiber drawing process is an important period of
optical fiber manufacturing.
[0005] A conventional drawing process of optical fiber is as
follows. A completed preform is fed slowly into a furnace where the
preform end is heated to about 2000.degree. C. and soften. The
soften glass forms a globule falling down from the furnace, through
the other stages of the drawing process and onto a take-up spool.
The stages thereof are outer diameter measurement, fiber cooling
stage, fiber coating stage, coating concentrity measurement, curing
stage, coating diameter measurement, fiber drawing capstans, proof
test and winding on take-up spool. The feeding speed depends on the
furnace design, preform diameter and draw speed. The optical fiber
which has just left the furnace and is remaining intact is called
"bare fiber". Usually, the required fiber diameter is controlled by
varying the fiber draw speed while keeping furnace temperature and
preform feeding speed constant. This is accomplished by monitoring
the fiber diameter immediately as it comes out of the furnace by
using one of several types of non-contacting methods, such as laser
light scattering (James J. Refi, Fiber optic Cable--A Light Guide).
Then, the controller uses this output signal of diameter
measurement to automatically adjust the speed of the drawing
capstans to obtain the correct output diameter.
[0006] The fiber cools down after leaving the diameter monitor.
There may be a cooling device or just a natural cooling stage.
Then, it has a protective plastic coating applied in order to
preserve strength, to isolate itself from external force and to
avoid microbending losses. There are two coating stages: one inner
soft primary coating and another outer hard secondary coating.
After coating applicator, a monitor measures concentricity which is
another important specification. Then, the fiber passes a curing
furnace/lamp. After coating and curing stages, a second diameter
monitor is used to measure coated fiber diameter, i.e., to provide
coating diameter measurement.
[0007] Next stage after coating and curing is to pull fiber on to a
take-up spool. The fiber passes drawing capstans and wounded on to
a take-up spool. There is a proof test, which may be an on-line
test before the fiber is placed on the spool or an off-line test
after the fiber is placed on the spool. The test may include a
strength and tension test. In this fiber take-up stage, winding
tension is kept low and the drum's surface is wrapped with a soft
foam to cushion the fiber from the drum's surface in order to
minimize the effect of added microbending losses. Refer to FIG. 11
to show this conventional drawing process (James J. Refi, Fiber
optic Cable--A Light Guide).
[0008] It is known to employ a control method in which variations
in the external diameter of the optical fiber are fed back to a
pulling capstan. FIG. 11 (Prior Art) is a schematic diagram of a
conventional drawing apparatus utilizing such a control method. An
optical fiber preform 2 is melted in a heating furnace 3, pulled by
a capstan 13, and formed into an optical fiber 5. The external
diameter of the optical fiber 5 is measured by an external diameter
measuring instrument 20 which is located just downstream of the
heating furnace 3. Thereafter, the optical fiber 5 is coated with a
resin by a coating unit 6, and then pulled through a resin curing
furnace 9.
[0009] The external diameter measurements from the external
diameter measuring instrument 20 are fed back to the control system
of the pulling rate of the pulling capstan 13 so that the optical
fiber has a consistent, predetermined external diameter (normally
125 .mu.m).
[0010] A diameter of the bare fiber is measured by a measuring
device 20 before the coating steps 6 and 9, whereby conditions
during the drawing are controlled so that the outer diameter of the
fiber 5 is to be a preselected one.
[0011] A position at which the diameter measuring device is
disposed has not been thought to be critical, and the device is
usually located immediately below the drawing furnace as shown in
Japanese Patent Kokai Publication No. 295260/1986.
[0012] The main reason to locate the measuring device close to the
furnace is to reduce the time lag and to increase a control gain
when fluctuation in the diameter of the optical fiber has to be
suppressed by controlling a drawing rate depending on an output
signal from this measuring device.
[0013] If there is anything to limit the position of the measuring
device, it has been that the measuring device should not be
directly subjected to a strong radiation light from a lower portion
of the furnace 3 to avoid being heated to a remarkably high
temperature, i.e., located at a safe place.
[0014] Thus, as mentioned in U.S. Pat. No. 5,073,179/1991, in the
conventional production of the optical fiber, usually a distance
between the outer diameter measuring device 20 and the coating die
6 is longer than that between the drawing furnace 3 and the
measuring device 20, or a forced cooling device is disposed between
the measuring device 20 and the coating die 6 in order to achieve a
better resin coating.
[0015] It has been noticed that the drawing rate of the optical
fiber in drawing process is remarkably increased from a
conventional drawing rate with the order of 100 m/min and it is
reported that, in an experimental scale, a rate of 1000 m/min has
been realized. When such a high drawing rate is employed in the
conventional process in which the measuring device is located
immediately below the furnace, it has been found that the outer
diameter of the finished optical fiber is extremely smaller than
the diameter which is measured with the measuring device 20. In
order to meet an accuracy requirement in optical fiber diameter,
U.S. Pat. No. 5,073,179/1991 proposes a modified fiber drawing
process (see FIG. 10 Pre Art) wherein an outer diameter of the
optical fiber 5 on which no coating has been provided is measured
at position 30 at which shrinkage of the outer diameter of the
optical fiber, while stretched, is not larger than 0.5% preferably
0.5 to 0.3% and drawing conditions are controlled with a deviation
of a measured diameter at position 30 from a preselected outer
diameter.
[0016] As used herein, the term "shrinkage" is intended to mean a
ratio of difference in diameters between the optical fiber at the
measuring position and the optical fiber once it has finished
shrinking.
[0017] For example, the accuracy of the diameter of a quartz base
optical fiber is usually required to be in 125 .mu.m.+-.1 .mu.m.
Taking account into an accuracy of the measuring device itself and
the fluctuation in the diameter of the optical fiber during the
production, a deviation of the measured diameter with the measuring
device from a true diameter of the finished fiber should be not
larger than 0.5% of the outer diameter of the finished fiber. Thus,
a desirable process has been developed with a goal to reach the
deviation of 0.5% or less in U.S. Pat. No. 5,073,179/1991.
[0018] The position at which the measuring device 30 is disposed is
determined on the basis of the estimation of the fiber temperature
according to the following equation (1) and experiments:
T(Z)=T(O)+(T(S)-T(O))e.sup.-aZ/V(F) (1)
[0019] where T(O) is a room temperature (.degree.C.), T(S) is a
temperature (.degree.C.) of an optical fiber immediately after
leaving a furnace, Z (m) is a distance from an outlet of the
furnace to a position at which an outer diameter of the optical
fiber is measured, V(F) is a drawing rate (or linear velocity)
(m/min) and "a" is a constant determined with the diameter, a
specific heat of the optical fiber and a thermal conductivity
between the optical fiber and an atmosphere.
[0020] For the experiments, first, the difference between the
measured outer diameter at position 30 in FIG. 10 and the true one
is obtained with varying the position of the measuring device.
Then, a relation between the difference and the measuring position
30 is established. Finally, the position 30 is determined at which
the difference is less than 0.5%. Thus, the measuring device can be
located at that position 30 and an optical fiber having a better
accuracy is produced. It provides a better results comparing with
early conventional method. However, it may not robustly maintain a
high accuracy due to many disturbance factors, perturbations, and
environment changes, as well as drawing speed level changes
mentioned in the following paragraph.
[0021] It also takes time to detect the outer diameter of the fiber
which is under increase in its diameter in the case of small
drawing rate, whereby a time lag arises in the control. When the
drawing rate is small, detection of the outer diameter is carried
out with the measuring device at one location 31 nearer to the
furnace and when the rate is increased, the detection is carried
out with the measuring device at another location 32 below the
former location. Alternatively, only one measuring device is used
which can move along the optical fiber depending on the drawing
rate. It was described in U.S. Pat. No. 5,073,179/1991.
[0022] Further, a forced cooling device for the optical fiber is
provided between the furnace and the outer diameter measuring
device, whereby the distance between them can be shortened. In this
case, the diameter of the optical fiber is also measured at a
position 30 at which the shrinkage of the outer diameter is not
larger than 0.5%. When the drawing rate is largely exceeds 300
m/min, such a construction is especially suitable since large
scaling of the apparatus can be avoided and a prompt response can
be obtained.
[0023] As described above, the absolute value of the outer diameter
of the optical fiber which has been shrunk is measured within a
preselected shrinkage range 0.5%, whereby the optical fiber with
better accuracy in its size is produced. However, it is noticed
that the diameter of the optical fiber is still not robustly
controlled.
[0024] However, furthermore, from equation (1), it is noticed that
the faster the drawing speed V(F) is, the higher the temperature of
the optical fiber at a fixed position Z is. It is also noticed that
when the drawing speed increases, i.e., from 100 m/min to 300
m/min, the position for the outer diameter measuring device is
changed in this method.
[0025] Thus, necessary adjustment is needed to find the position at
which the measuring device is disposed within the preselected
shrinkage range 0.5%.
[0026] U.S. Pat. No. 5,443,610/1995 recognizes that the first
measurement of the fiber diameter made on bare fiber in the region
of the preform root, i.e., below the furnace, using an interference
technique is not sufficient for a good control for the fiber
diameter in view of error due to the fluctuation in the root with
tractor speed. Different from U.S. Pat. No. 5,073,179 moving the
diameter measuring device further from the furnace as the speed
increased, U.S. Pat. No. 5,443,610 suggests to combine this first
measured signal together with a second diameter measurement of a
coated fiber after coating device into an overall control signal
for drawing speed control in order to reduce the effect of such
error on the overall control signal. However, it is still lack of a
high accuracy measurement of the finished bare fiber diameter in
both U.S. Pat. Nos. 5,333,610 and 5,073,179.
[0027] It is noticed that when the external diameter of the optical
fiber varies due the presence of contaminants, such as microscopic
particulate matter, the variation in external diameter is detected
by the external diameter measuring device.
[0028] However, if bubbles or the like remain in the optical fiber
preform over some length as it is drawn, the pulling rate of the
pulling capstan 13 is suitably altered to maintain a consistent
external diameter of the optical fiber 5. Accordingly, the external
diameter measuring instrument 20 cannot detect irregular portions
of the optical fiber 5. However, the bubbles may collapse when the
optical fiber is drawn, even though the external diameter is
controlled. Consequently, the core of the optical fiber deforms and
causes increased transmission loss. U.S. Pat. No. 5,449,393/1995
uses a feedback control based on detecting irregular ratio of
pulling rate to a predetermined time period.
[0029] To reduce a lag, the measurement device 20 is located
immediately below the furnace; while to reduce the inaccuracy of
outer diameter due to a shrinkage, the measurement device 30 is
located within a shrinkage range of 0.5% with an estimate or
experiments in U.S. Pat. No. 5,073,179.
[0030] In response to a hot market demand, the fiber drawing
process becomes a high speed process to make high productivity. The
preform becomes larger and larger and the drawing speed of the
optical fiber in the drawing process becomes faster and faster.
When preform becomes larger and larger, a unit deviation of the
diameter of a preform from the preselected diameter will bring a
larger area deviation and further a larger mass deviation of the
preform from the preselected form.
[0031] Due to many physical and chemical factors, and environment
changes, the preforms have fluctuations in their outer diameters
within one preform and among different preforms. Thus, the
diameters of various performs and the diameters of one perform at
different locations are not uniform.
[0032] It is known that two major origins of fiber diameter
fluctuation are temperature fluctuation of the furnace and the
preform outer diameter fluctuation. The former origin usually
causes short term fluctuation and the latter usually causes long
term fluctuation of the fiber diameter fluctuation. Thus, it is
important to know the preform diameter for the capstan drawing
speed control and the preform feeding mechanism control in the
drawing process control system in order to get a perfect diameter
for the bare fiber. However, there are no preform diameter
measurement step and its device during the conventional optical
fiber drawing processes currently.
[0033] In order to compromise the reduction of the time lag and the
accurate measurement of the fiber outer diameter, conventional
methods set the outer diameter measurement of the bare fiber either
near the furnace, or within the 0.5% shrinkage range, or at an
adjustable position keeping the shrinkage range but as near the
furnace as possible for the different drawing speeds.
[0034] It is still critical to find a novel method to solve both
the measurement lag problem and the shrinkage problem. In other
words, conventional methods must be at the sacrifice of either
accurate measurement or prompt reaction due to only one outer
diameter monitor 20 or 30 for the bare fiber 5.
[0035] Furthermore, it should be noticed that no convention method
intends to monitor a finished bare fiber diameter which is really
an important specification, e.g., 125 .mu.m, before the coating
step. However, in order to control exact outer diameter of the bare
fiber with the highest accuracy, it is not only important but also
necessary from the viewpoint of control system to measure the final
outer diameter of the bare fiber on-line for generating feedback
signals to the whole control system.
[0036] As the requirements for accuracy in optical fiber diameter
increase and the minimization of fluctuation in that diameter
become critical as the result of the connection between fibers
becoming better, development of a process which improves the
accuracy of the outer diameter of the optical fiber is highly
desired. In view of the material fluctuation, perturbations of
preforms, disturbances in the furnace, and environment changes, the
optical fiber needs a robust optical fiber drawing process control
to produce robust diameter-controlled optical fiber.
[0037] In all, current conventional fiber drawing processes are
lack of robust control for the fiber diameter.
BRIEF SUMMARY OF THE INVENTION
[0038] It is an objective of this present invention to provide a
highly robust diameter-controlled optical fiber.
[0039] Further, it is an object of the present invention to provide
a drawing process for producing an optical fiber in which a high
accuracy of an absolute value of an optical fiber diameter is
ensured. Especially, a deviation of a measured outer diameter with
a diameter measuring device from a true diameter of a finished
optical fiber can be minimized, which is much smaller than that as
obtained in the conventional processes.
[0040] It is found that an outer diameter measurement at a safe
position just immediately above the coating device provides not
only an exact on-line measurement for the outer diameter of the
finished bare fiber, but also an important feedback signal for
drawing process control to adjust its capstan speed because the
finished bare fiber diameter is a required control variable and an
important required specification. The position for this measurement
does not need any adjustment for different drawing speeds. Thus, it
avoids shrinkage ratio calculation and experiments for searching a
suitable position to locate this outer diameter monitor. Only
consideration needed for the monitor position is that it is nearest
to coating device and safe for the monitor. Finally, this
measurement provides the highest accurate outer diameter
measurement of the finished bare fiber among any possible
measurements above the coating device. It minimizes the difference
between the measurement and the exact/true outer diameter of the
finished bare fiber.
[0041] In control viewpoint, it is necessary to measure the final
outer diameter of the bare fiber in order to control exact outer
diameter of the bare fiber, and then to use the deviation of this
exact bare fiber diameter from the preset diameter for feedback
control. It means that a measurement should locate at a position
just before the coating step. However, no current conventional
methods have mentioned this way for the outer diameter measurement
of the bare fiber.
[0042] In order to solve the time lag problem and the highest
accurate measurement requirement for optical fiber, the present
invention keeps a conventional outer diameter measurement at a safe
position just immediately below the furnace. The reason for this
measurement is that this measurement has the smallest time lag
among any possible measurements below the furnace in order to use
this measurement signal for the fiber drawing process control in
face of the temperature fluctuation, the humidity fluctuation and
dust particles in the furnace.
[0043] The present invention has a new outer diameter measurement
of the preform at a position before the melting step in the
furnace, usually at a safe position just immediately above the
furnace. As the preform becomes larger and larger, this measurement
becomes much more important to robustly control optical fiber
diameter accuracy, especially, in view of that the preform outer
diameter fluctuation is one major origin of fiber diameter
fluctuation.
[0044] This new preform outer diameter monitor provides useful
information of the preform diameters not only regarding the
fluctuation of preform outer diameter but also regarding the end
shape of the preform for the fiber drawing speed control and the
preform feed mechanism control during the optical fiber drawing
process.
[0045] In the present invention, the measurement signals from the
preform outer diameter, the bare fiber diameter just leaving the
furnace and the finished bare fiber diameter just before coating
are fed back to the optical fiber drawing process control system to
robustly control the drawing speed of the capstan and the feed
speed of the preform feed mechanism.
[0046] According to the present invention, there is provided a
drawing process for producing an optical fiber which comprises
drawing the optical fiber from an outer diameter monitored preform
therefor under tension to form the optical fiber while heating and
melting the preform, wherein the outer diameter of the optical
fiber on which no coating has been provided is measured at two
different process positions, one of which is at a safe position
immediately below the furnace and another of which is at a safe
position immediately above the coating device where the bare fiber
is finished, and drawing conditions are robustly controlled based
on the measurement data from all these measurement instruments.
[0047] In the present invention, one choice of control law can be,
but not limited to, based on a deviation of a final measured
diameter of the bare fiber coming into the coating step from the
specified outer diameter, a deviation of a measured diameter of the
bare fiber leaving from the furnace from a preselected outer
diameter, and a deviation of a measured outer diameter of the
preform coming into the furnace from a preselected outer diameter.
It may also be based on the fluctuation data from the current
measurements in the above-mentioned diameter measurement monitors
and the ones as certain period measurement history data.
[0048] In one preferred embodiment of the present invention, a
temperature of the optical fiber at the measuring position of the
outer diameter of the bare fiber which is just above the coating
device is much lower than the glass softening point of the material
of the optical fiber.
[0049] In another preferred embodiment of the present invention, a
drawing rate (or linear velocity) from the preform is varied
depending on the deviations in order to control the outer diameter
of the optical fiber.
[0050] Variations of the present invention can include a
combination of any partial invention in the present invention and
any current conventional optical fiber drawing method or any
mixture of current conventional optical fiber drawing method.
[0051] When twice outer diameter measurements are selected between
the drawing furnace and the coating device, the first measurement
device can be located at a safe position immediately after the
furnace, and the second measurement device can be located at a safe
position immediate above the coating device. When one outer
diameter measurement is selected between the furnace and coating
device, the measurement device may be located at an adjustable
position, either at a position immediately before the coating step,
or at a position at which shrinkage of the outer diameter of the
optical fiber, while stretched, is not larger than the bare fiber
diameter accuracy requirement, or at a safe position immediately
below the drawing furnace.
[0052] This present invention includes a control for optical fiber
drawing process based on the all or partial above-mentioned
measurement data and/or the all or partial conventional measurement
data, e.g., from an outer diameter measurement device located at a
position at which shrinkage of the outer diameter of the optical
fiber, while stretched, is not larger than 0.5% preferably 0.5 to
0.3%, or from an outer diameter measurement device located
immediately below the furnace, during the optical fiber drawing
process. Thus, it is a kind of robust control in face of
fluctuations of the preform outer diameters and fluctuation of the
bare fiber outer diameters due to various factors including the
furnace temperature fluctuation,
[0053] The present invention provides a very high quality of
optical fiber with robustly controlled diameter of the optical
fiber with very high diameter accuracy.
[0054] This present invention can also improve the strength of the
optical fiber in view of a robustly controlled diameter of the
optical fiber and robustly smooth drawing process due to the
contribution to the control system from the above-mentioned
invented on-line outer diameter measurements of the bare fiber and
the preform.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0055] Listing of all figures:
[0056] 1. A schematic of robust diameter-controlled optical fiber
drawing process;
[0057] 2. Another schematic of robust diameter-controlled optical
fiber drawing process;
[0058] 3. A schematic of robust diameter-controlled optical fiber
drawing process with one bare fiber diameter monitor;
[0059] 4. Another schematic of robust diameter-controlled optical
fiber drawing process with one bare fiber diameter monitor
considering shrinkage;
[0060] 5. Another schematic of robust diameter-controlled optical
fiber drawing process with one bare fiber diameter monitor
considering shrinkage at an adjustable position;
[0061] 6. A schematic of robust diameter-controlled optical fiber
drawing process with only one bare fiber diameter monitor near the
furnace;
[0062] 7. A schematic of robust diameter-controlled optical fiber
drawing process with two bare fiber diameter monitors;
[0063] 8. Another schematic of robust diameter-controlled optical
fiber drawing process with two bare fiber diameter monitors;
[0064] 9. A schematic of diameter-measured optical fiber drawing
process;
[0065] 10. A schematic of optical fiber drawing process (Pre Art
1);
[0066] 11. Another schematic of optical fiber drawing process (Pre
Art 2).
[0067] FIG. 1 is a schematic diagram showing one preferred
embodiment of the present invention, in which an outer diameter
measurement device is located immediately above the furnace and
provides the outer diameter data of the preform to the control
system; an outer diameter measurement device is located immediately
below the furnace and provides the outer diameter data of the bare
fiber to the control system; and an outer diameter measurement
device is located immediately above the coating device and provides
the outer diameter data of the finished bare fiber to the control
system.
[0068] FIG. 2 is a schematic diagram showing another embodiment of
the present invention, in which an outer diameter measurement
device is located immediately above the furnace and provides the
outer diameter data of the preform to the control system; an outer
diameter measurement device is located immediately below the
furnace and provides the outer diameter data of the bare fiber to
the control system; and an outer diameter measurement device is
located within a shrinkage range of 0.5%.about.0.3% above the
coating device and provides the outer diameter data of the bare
fiber with estimated finished diameter error range within
0.5%.about.0.3% to the control system.
[0069] FIG. 3 is a schematic diagram showing a further embodiment
of the present invention, in which an outer diameter measurement
device is located immediately above the furnace and provides the
outer diameter data of the preform to the control system; and only
one bare fiber outer diameter measurement device is located
immediately above the coating device and provides the outer
diameter of the finished bare fiber just coming into coating step
to the control system.
[0070] FIG. 4 is a schematic diagram showing a further embodiment
of the present invention, in which an outer diameter measurement
device is located immediately above the furnace and provides the
outer diameter data of the preform to the control system; and only
one fiber outer diameter measurement device is located at a
position within a shrinkage range of 0.5%.about.0.3% between the
furnace and the coating device and provides the outer diameter data
of the bare fiber with estimated finished diameter error range
within 0.5%.about.0.3% to the control system.
[0071] FIG. 5 is another schematic diagram showing a further
embodiment of the present invention, in which an outer diameter
measurement device is located immediately above the furnace and
provides the outer diameter data of the preform to the control
system; and one fiber outer diameter measurement device is located
at a different position 31 or 32, in which, when the drawing rate
is small, detection of the outer diameter is carried out with the
measuring device 31 and when the rate is increased, the detection
is carried out with the measuring device 32. Alternatively, only
one measuring device is used which can move along the optical fiber
depending on the drawing rate.
[0072] FIG. 6 is a schematic diagram showing another embodiment of
the present invention, in which an outer diameter measurement
device is located immediately above the furnace and provides the
outer diameter data of the preform to the control system; and only
one outer diameter measurement device is located immediately below
the furnace and provides the outer diameter of bare fiber just out
from the furnace to the control system.
[0073] FIG. 7 is a schematic diagram showing one embodiment of the
present invention, which includes two on-line outer diameter
measurement devices: one outer diameter measurement device is
located immediately below the furnace and provides the outer
diameter data of the bare fiber to the control system; and another
outer diameter measurement device is located immediately above the
coating device and provides the outer diameter data of the finished
bare fiber to the control system.
[0074] FIG. 8 is a schematic diagram showing one embodiment of the
present invention, which has two on-line outer diameter measurement
devices between the furnace and the coating device: one outer
diameter measurement device is located immediately below the
furnace and provides the outer diameter data of the bare fiber to
the control system; and another outer diameter measurement device
is located at a position with a shrinkage range of 0.5%.about.0.3%
and provides the outer diameter data of the bare fiber with
estimated finished diameter error range within 0.5%.about.0.3% to
the control system.
[0075] FIG. 9 is a schematic diagram showing a variation of an
embodiment of the present invention, which includes only one
on-line outer diameter measurement device which is located
immediately above the coating device and provides the outer
diameter data of the finished bare fiber to the control system.
[0076] FIG. 10 (Prior Art) is a schematic diagram illustrating an
improved conventional optical fiber drawing process (ref. to U.S.
Pat. No. 5,073,179).
[0077] FIG. 11 (Prior Art) is a schematic diagram illustrating a
conventional optical fiber drawing process (ref. to James J. Refi,
Fiber optic Cable--A Light Guide).
DETAILED DESCRIPTION OF THE INVENTION (PREFERRED EMBODIMENTS)
[0078] The requirement for a high quality fiber drawing process
includes the followings:
[0079] A uniform fiber diameter as industry requires, e.g., usually
125 .mu.m for the glass;
[0080] A uniform coating making a coated fiber diameter such as of
250 .mu.m;
[0081] Concentricity of fiber glass and coating layers;
[0082] A constant tension to satisfy the mechanical strength and
avoid microbending; and
[0083] A high speed process to make high productivity.
[0084] The optical fiber diameter is very important to reducing the
loss of light transmission. It is an important issue to get high
grade optical fiber with the specified optical fiber diameter and
needed accuracy of its optical fiber diameter. Most fibers produced
have a diameter of 125 .mu.m with deviations of less than 1 .mu.m
required for cabling and splicing. Certainly, the smaller the
deviations of the optical fiber diameter, i.e., the higher accuracy
of the optical fiber diameter, the better the performance of
optical fiber is. The optical fiber diameter is effected by many
factors during the optical fiber drawing process. The main factors
include the furnace temperature fluctuation, the preform diameter
fluctuation, the drawing speed by the capstan, and the preform
feeding speed. It is well know that in the stable fiber drawing
process, the resulting size of the bare fiber relative to the size
of the preform is determined by the drawing speed of the fiber
relative to the feed rate of the preform as described in the
following equation (2):
.nu..sub.d=.nu..sub.f.multidot.(D.sup.2/d.sup.2) (2)
[0085] where .nu..sub.d is the fiber drawing speed, .nu..sub.f is
the preform feeding speed, D is the preform diameter and d is the
fiber diameter. It is obvious from equation (2) that the preform
diameter is one major factor for determining the fiber drawing
speed.
[0086] Thus, it is important to have a preform diameter monitor to
measure the preform diameter D for securing a high accuracy of the
finished optical fiber diameter d during the optical fiber drawing
process. It is a main part of this present invention for the robust
diameter-controlled optical fiber.
[0087] Further, it is noticed that due to the above-mentioned
requirement of a high speed drawing process for a high
productivity, the preforms become larger and larger in both
diameter and length. Suppose a fluctuation .DELTA.D in a preform
diameter. In order to maintain the exact same diameter d of the
optical fiber, the fiber drawing speed should have an adjustment
amount .DELTA..nu..sub.d and the preform feeding speed should have
an adjustment amount .DELTA..nu..sub.f as follows:
.DELTA..nu..sub.d=[.nu..sub.f.multidot.(2D.multidot..DELTA.D+.DELTA.D.sup.-
2)+.DELTA..nu..sub.f.multidot.(D+.DELTA.D).sup.2]/d.sup.2 (3)
[0088] which is derived from equation (2). Thus, when the preform
diameter becomes larger, a same deviation .DELTA.D needs a larger
adjustment .DELTA..nu..sub.d of the fiber drawing speed or a larger
adjustment .DELTA..nu..sub.f of the preform feeding speed in order
to maintain the exact required outer diameter d of the optical
fiber. It proves an urgent requirement for a preform diameter
monitor during the fiber drawing process in presence of fluctuation
of the preform diameters when the preform diameters become
larger.
[0089] The location in which the fiber is formed and is mainly
changing its size is the furnace zone. Thus, the position to locate
a preform diameter monitor should be as near to the furnace as
possible. If there is anything to limit the position of the
measuring device, it has been that the measuring device should not
be directly subjected to a strong radiation light from an upper
portion of the furnace to avoid being heated to a remarkably high
temperature. This arrangement should reduce the lead-time of the
preform diameter data comparing the diameter data at a position in
the furnace where the glass is melting and drawing into the fiber.
For a high accuracy control, it may save a memory size in the
control system.
[0090] This preform diameter measurement signal is fed back to both
capstan speed control system and preform feeding speed control
system during the optical fiber drawing process.
[0091] As above mentioned in the summary part, in order robustly to
control the finished optical fiber diameter with a very high
accuracy, it is necessary to measure the final optical fiber
diameter just before the coating step. The reasons are the
followings:
[0092] 1. First, it is the exact finished optical fiber diameter
providing an exact bare optical fiber diameter record for the
optical fiber products;
[0093] 2. Second, it can avoid any conventional "ahead" diameter
measurement estimation error for the exact diameter of the optical
fiber;
[0094] 3. Third, it can further avoid any measurement error subject
to the any effects due to contaminants, bubbles or the like remain
in the optical fiber preform over some length as it is drawn,
shrinkage, and so on.
[0095] This exact finished bare fiber diameter measurement is
another part of the present invention for robustly diameter
controlled optical fiber.
[0096] Because the main diameter change zone for the preform root
in forming the optical fiber is the heating zone of the furnace,
therefore it is better to have an outer diameter measurement
monitor immediately below the furnace in order to reduce signal lag
for the control. Thus, if there is anything to limit the position
of the measuring device, it has been that the measuring device
should not be directly subjected to a strong radiation light from a
below portion of the furnace to avoid being heated to a remarkably
high temperature.
[0097] It is the best to consider all three measurement signals
from the preform diameter measurement, the first bare fiber
diameter measurement and the second finished bare fiber diameter
measurement for generating a fiber drawing speed control signal and
a preform feeding control signal.
[0098] It can also been seen that to combine the above mentioned
first bare fiber diameter measurement and second finished bare
diameter measurement signals alone into an overall control signal
is better than the method to combine the first bare fiber diameter
measurement and the second coated fiber diameter measurement into
an overall control signal which described in U.S. Pat. No.
5,443,610 in order to robustly control the optical fiber glass
diameter, i.e., the final bare fiber diameter.
[0099] The preferred embodiments of the present invention are
described in the following figures with various combinations of the
three above-mentioned measurement monitors to produce robust
diameter-controlled optical fiber during optical fiber drawing
process.
[0100] FIG. 1 shows one most preferred embodiment of the present
invention, in which the numerical number 1 indicates the preform
feeding mechanism; 2 does the preform for the optical fiber; 3 does
a drawing furnace; 4 does a fiber cooling distance or a fiber
cooling device; 5 does a bare fiber between the furnace and the
coating device; 6 does a die for resin coating applicator; 7 does a
coated fiber; 8 does a coating concentrity monitor; 9 does a curing
device (a furnace or lamps) for the resin; 11 does a final coated
fiber; 12 does a coating diameter monitor; 13 does fiber drawing
capstans; 14 does proof test (e.g., strength test); 15 does a
winding up device for the optical fiber; 10 does a preform diameter
monitor; 20 does an outer diameter monitor for the bare fiber; and
40 does an outer diameter monitor for the finished bare fiber.
[0101] The preform 2 which is heated and melted in the furnace 3 is
stretched under tension to form the optical fiber 11, which is
drawn by capstans 13 and is taken up to spool 15 installed in the
winding up device.
[0102] In FIG. 1, monitor 10 for the preform is located at a safe
position immediately above the furnace 3 in order to reduce the
time lead; monitor 20 for the bare fiber is located at a safe
position immediately below the furnace 3 in order to reduce the
time lag; and monitor 40 for the finished bare fiber is located at
a safe position immediately above the coating device in order to
provide very high accurate measurement of the outer diameter for
the finished bare fiber.
[0103] Generally, the outer diameter of the preform is gradually
reduced in the furnace corresponding to an axial change of the
preform temperature (therefore, a viscosity change of the preform
material). Further, a size of the shrinking portion of the preform
depends on the drawing rate. As the drawing speed increases, the
outer diameter of the bare fiber and the fiber temperature at the
outlet of the furnace increase. Of course, to control the outer
diameter of the finished optical fiber depends on a preform
diameter, structural factors of the drawing furnace such as a
heating zone length, a size of the furnace outlet, a flow rate and
a kind of an inert gas, as well as a bare fiber diameter. Thus,
during the fiber drawing process, monitor 10 measures the outer
diameter of the preform which is coming into the furnace, monitor
20 measures the outer diameter of the bare fiber which is coming
out from the furnace, and monitor 40 measures the outer diameter of
the finished bare fiber which is coming into the coating
device.
[0104] Then, all these measured data are combined into an overall
robust control system to form a fiber drawing speed control signal
for capstans 13 and a preform feeding speed signal for the preform
feeding mechanism 1, respectively.
[0105] Thus, an optical fiber having a better accuracy of the outer
diameter of the optical fiber is produced. This outer diameter of
the optical fiber is robustly controlled.
[0106] It can be seen that the present invention is particularly
effective in the drawing of the optical fiber at a very high
drawing rate which is becoming faster and faster.
[0107] Another embodiment of the present invention is shown in FIG.
2, in which three outer diameter monitors above the coating device
are monitor 10 for the preform diameter measurement at a position
safely and immediately above the furnace 3, monitor 20 for the bare
fiber diameter measurement at a position safely and immediately
below the furnace 3, and monitor 30 for the second measurement of
the bare fiber at a position at which the diameter shrinkage range
less than 0.5%. This embodiment is suitable for improving some
conventional methods which currently have an outer diameter monitor
30 at that position with a shrinkage less than 0.5% as said in U.S.
Pat. No. 5,073,179. The robust control system obtains the feedback
signals from these diameter monitors and controls the fiber drawing
speed and the preform feeding speed for producing robustly diameter
controlled optical fibers.
[0108] The third embodiment of the present invention is shown in
FIG. 3, in which two outer diameter monitors above the coating
device are monitor 10 for the preform diameter measurement at a
position safely and immediately above the furnace 3, and monitor 40
for the outer diameter measurement of the finished bare fiber at a
position safely and immediately above the coating device. They
provide control system feedback signals to the fiber drawing
process control system for producing high quality optical
fibers.
[0109] The fourth embodiment of the present invention is shown in
FIG. 4, in which two outer diameter monitors above the coating
device are monitor 10 for the preform diameter measurement at a
position safely and immediately above the furnace 3, and monitor 30
at a position with a shrinkage ratio less than 0.5%. The control
system collects these measurement data and determines the fiber
drawing speed and the preform feeding speed.
[0110] A further embodiment of the present invention shown in FIG.5
is to let monitor 30 in FIG.4 can be adjusted based on the drawing
speed, high or low. In the embodiment as shown in FIG. 4, it takes
time to detect the outer diameter of the fiber which is under
increase in its diameter in the case of small drawing rate, whereby
a time lag arises in the control. In the embodiment as shown in
FIG. 5, when the drawing rate is small, detection of the outer
diameter is carried out with the measuring device 31 and when the
rate is increased, the detection is carried out with the measuring
device 32. Alternatively, only one measuring device is used which
can move along the optical fiber depending on the drawing rate. The
rest part in FIG.5 is the same as the corresponding rest part in
FIG.4, such as monitor 10.
[0111] The sixth embodiment of the present invention is shown in
FIG. 6, in which two outer diameter monitors above the coating
device are monitor 10 for the preform diameter measurement at a
position safely and immediately above the furnace 3, and monitor 20
for the outer diameter measurement of the bare fiber at a position
safely and immediately below the furnace 3. The control system
collects these measurement data and determines the fiber drawing
speed and the preform feeding speed.
[0112] Monitor 20 has the smallest time lag than monitor 30 (or 31
or 32), especially when the drawing speed is low. Monitor 30
provides a higher accuracy of the out diameter of the optical fiber
than monitor 20 when the drawing speed is high. However, monitor 40
has the highest accuracy of outer diameter measurement for the
finished bare fiber, i.e., the optical fiber glass diameter.
[0113] A further variation of embodiments of the present invention
is shown in FIG. 7, in which two outer diameter monitors between
the furnace 3 and the coating device 6, but no diameter monitor for
the preform diameter measurement above the furnace 3. Here, one
monitor is monitor 20 and another is monitor 40 at the same
respective positions as above-mentioned. This method can solve the
problem which described in U.S. Pat. No. 5,443,610 in a better way
since the monitor 40 can provide a higher accurate measurement of
the outer diameter of the finished bare fiber with a less lag time
than monitor 12 can in the conventional methods.
[0114] FIG. 8 shows another variation of embodiments of the present
invention. In FIG.8 monitor 20 and monitor 30 or 31 or 32 have been
selected. They are at the positions as above-mentioned
respectively. This method can provide the control system a less
time lag information of the outer diameter of bare fiber from
monitor 20 in addition to the information provided from monitor 30
or 31 or 32. It is really a new method to combine two previous
separated conventional methods in order to get a more accurate
outer diameter for optical fibers than any one of these two
conventional methods individually.
[0115] FIG. 9 shows a further variation of embodiments of this
present invention. In FIG. 9 the process has only one diameter
monitor 40 between the furnace 3 and the coating device 6. This
method may be an alternative method with an advantage of accurate
measurement of the outer diameter of the finished bare fiber and
keeping a record of this specification for the product optical
fiber. However, this method has a cost of time lag for the control
system.
[0116] Finally, FIG. 10 and FIG. 11 show two conventional methods
of optical fiber drawing process respectively.
[0117] FIG. 10 has only monitor 30 (or 31 or 32) for measuring
outer diameter of bare fiber at a position at which the shrinkage
of the outer diameter is not larger than 0.5% (U.S. Pat. No.
5,073,179).
[0118] FIG. 11 shows only monitor 20 for measuring outer diameter
of bare fiber at a conventional position immediately below the
furnace 3. In this location monitor 20 minimizes the time lag with
a cost without an accurate measurement of the outer diameter for
the final bare fiber.
[0119] Finally, it should be pointed out that for the
above-mentioned embodiments of the present invention shown in FIGS.
1-9, a forced cooling device for the optical fiber may be provided
between furnace 3 (or monitor 20 when it is selected in the
embodiment) and coating device 6 (or outer diameter measuring
device 40 or 30 when it is selected in the embodiment), whereby the
cooling distance between them can be shortened and the time lag can
be shortened as well. Such a construction is especially preferred
since large scaling of the apparatus can be avoided and a prompt
response can be obtained.
[0120] As described above, according to the present invention, the
absolute value of the outer diameter of the optical fiber can be
not only measured correctly but also robustly controlled, whereby
the optical fiber with better accuracy in its size is produced in
face of fluctuations of the preform diameters, fluctuations of the
furnace conditions, various disturbances and parameter
perturbations, especially in a high productivity situation with
increasing drawing speed, enlarging preform size and raising high
performance of optical fiber during the optical fiber drawing
process. Thus, this present invention provides robust
diameter-controlled optical fibers during optical fiber drawing
process.
* * * * *